Heat pumps are used to bring heat at a low temperature to a higher (usable) temperature level e.g. heat from the ground or groundwater to be raised to a usable temperature level for under-floor heating. Commercial systems are so-called single stage heat pumps, see FIG. 1. Between the evaporator and the condenser there is one stage (one compressor and one expansion valve).
FIG. 2 shows a theoretical plot of log p versus h, where p is the pressure and h is the enthalpy, for a single stage heat pump cycle with a dome-like region, the so-called liquid-vapour dome, and the cycle for a single stage heat pump: the lower horizontal line with an arrow pointing to the right representing the evaporation step, followed by compression step, a condensation step (upper horizontal line with arrow pointing to the left) and finally expansion at constant enthalpy (vertical line with arrow pointing downwards). At lower enthalpies than those within the dome-like region (the so-called liquid-vapour dome) (i.e. to the left thereof) liquid exists with a mix of saturated liquid and saturated vapour in the dome-like region and vapour existing at higher enthalpies than those within the dome region (i.e. to the right thereof). The critical point is at the apex of the dome region with vapour existing to the non-dome-like area to the left thereof, a vapour existing in the non-dome-like area to the right thereof and a supercritical fluid existing above the critical point.
For larger temperature lifts, i.e. the difference between the temperature of the heat source and the output temperature of the heat pump, two stage heat pumps can be used comprising an additional intermediate pressure level, two compressors and two expansion valves. The advantage of two stages is that the pressure ratio which has to be realised by each of the compressors is halved compared with that for a single stage system. Furthermore, gas compressed in the first stage can be cooled, whereupon the density increases and the temperature of the gas at the second stage decreases. The performance of the second compression step can then be improved.
However, two stage systems have only been used for high temperature lifts because of the investment costs involved. Purely on the basis of energy considerations two stage systems are also of interest for lower temperature lifts. FIG. 3 shows a theoretical plot of log p versus h, where p is the pressure and h is the enthalpy, for a two stage heat pump cycle with the same liquid-vapour dome as for FIG. 2. Three stage systems are known for cryogenic applications. The greater the number of stages the higher will be the performance of a heat pump, but with the disadvantage that the investment required increases considerably.
GB 2049901A discloses a heat pump apparatus, comprising: a plurality of separate heat pump circuits, each of the said circuits being adapted to have a heat transferring fluid circulate therethrough and each including respective evaporator means and condenser means, and means for directing a mass flow to be heated into heat exchange relationship with each of the said condenser means in series, whereby the temperature of the mass flow to be heated rises when in heat exchange relationship with the fluids circulating through the condenser means of the respective heat pump circuits. To increase efficiency, a number of separate and continuously heat pump circuits are used, their compressors having a common drive, while the condensers are connected in series in relation to the current to be heated so as to cause its temperature to rise by heat-exchange with the media flowing in the pump circuits.